For more than three decades, the continued miniaturization of silicon metal oxide semiconductor field effect transistors (MOSFETs) has driven the worldwide semiconductor industry. Various showstoppers to continued scaling have been predicated for decades, but a history of innovation has sustained Moore's Law in spite of many challenges. However, there are growing signs today that metal oxide semiconductor transistors are beginning to reach their traditional scaling limits. A concise summary of near-term and long-term challenges to continued CMOS scaling can be found in the “Grand Challenges” section of the 2002 Update of the International Technology Roadmap for Semiconductors (ITRS). A very thorough review of the device, material, circuit, and systems can be found in Proc. IEEE, Vol. 89, No. 3, March 2001, a special issue dedicated to the limits of semiconductor technology.
Since it has become increasingly difficult to improve MOSFETs and therefore complementary metal oxide semiconductor (CMOS) performance through continued scaling, methods for improving performance without scaling have become critical. One approach for doing this is to increase carrier (electron and/or hole) mobilities. This can be done by either: (1) introducing the appropriate strain into the Si lattice; (2) by building MOSFETs on Si surfaces that are orientated in directions different than the conventional <100> Si; or (3) a combination of (1) and (2).
As far as approach (1) is concerned, the application of stresses or strains changes the lattice dimensions of the Si-containing layer. By changing the lattice dimensions, the energy band gap of the material is changed as well. The change of effective mass and band structure due to stress enhances or reduces electron and/or hole mobility.
N-type channel field effect transistors (nFETs) need tension on the channel for strain-based device improvements, while p-type channel field effect transistors (pFETs) need a compressive force on the channel for strain-based device improvements.
In terms of approach (2), electrons are known to have a high mobility for a (100) Si surface orientation, but holes are known to have high mobility for a (110) surface orientation. That is, hole mobility values on (100) Si are roughly 2× lower than the corresponding electron hole mobility for this crystallographic orientation. To compensate for this discrepancy, pFETs are typically designed with larger widths in order to balance pull-up currents against the nFET pull-down currents and achieve uniform circuit switching. nFETs having larger widths are undesirable since they take up a significant amount of chip area.
On the other hand, hole mobilities on the (110) crystal plane of Si are approximately 2× higher than on the (100) crystal plane of Si; therefore, pFETs formed on a surface having a (110) crystal plane will exhibit significantly higher drive currents than pFETs formed on a surface having a (100) crystal plane. Unfortunately, electron mobilities on the (110) crystal plane of Si are significantly degraded compared to the (100) crystal plane of Si.
There is interest in integrating strained silicon having multiple crystallographic orientations with silicon directly on insulator (SDOI) technology. SDOI substrates reduce parasitic capacitance within the integrated circuit, reduce individual circuit loads and reduce the incidence of latch-up, thereby improving circuit and chip performance.
In view of the state of the art mentioned above, there is a continued need for providing a strained Si directly on insulator (SSDOI) substrates with multiple crystallographic orientations and with the same or different stress levels.